Cylindrical, prismatic or coin batteries each include a battery case made of metal or the like, an electrode assembly and an electrolyte accommodated in the battery case, the battery case being sealed at its opening with a sealing plate. The positive electrode and the negative electrode are wound or stacked with a separator interposed therebetween, forming an electrode assembly. The separator provides electrical insulation between the positive electrode and the negative electrode, and retains an electrolyte.
In cylindrical or coin batteries, the sealing plate has a polarity different from the polarity of the battery case. Because of this, a resin gasket is interposed between the sealing plate and the opening-edge portion of the battery case, providing sealing of the battery case as well as insulation between the sealing plate and the battery case.
In prismatic batteries, the sealing plate has the same polarity as the polarity of the battery case. The sealing plate has a through hole at the center thereof, and into the through hole, a terminal member having a polarity different from the polarity of the battery case is inserted. A gasket is interposed between the sealing plate and the terminal member, providing insulation between the sealing plate and the terminal member.
In the event when an internal short circuit or external short circuit occurs, and a short circuit current is caused to flow to generate Joule heat in a battery, the battery will generate heat and may become overheated. In particular, with respect to lithium ion secondary batteries, because of their high energy densities, it is highly necessary to prevent overheating and increase the degree of safety. For this reason, various structures for preventing short circuit have been proposed.
In order to prevent an external short circuit, for example, one proposal suggests incorporating a PTC (Positive Temperature Coefficient) element, a thermal fuse or the like, which acts as a fail-safe device, in a battery or a circuit for battery-powered equipment. When an excessive amount of current flows due to short-circuiting between external terminals, and the battery temperature is elevated, the PTC element or thermal fuse is activated, to interrupt the current.
In order to prevent an internal short circuit, one proposal suggests forming an electrically insulating layer with thermal resistance between the positive electrode and the negative electrode (Patent Literature 1).
Further, as a method of preventing a short circuit between the battery case and the internal power generation element from occurring in the battery case, forming an electrically insulating layer in a portion of the inner surface of the battery case facing the electrode assembly is proposed (Patent Literatures 2 and 3).